Hmmmmm- - - -is this part a thin sheet metal type washer that fits on the inboard side of the bearing? If so, it's probably there to reduce flooding the bearing with rear end grease when the car is in a turn. A little grease will find its way past the washer and keep the bearing lubed, but most of it will get deflected back toward the differential. If the washers aren't damaged, reusing them won't be a problem. The axle shaft shouldn't contact them at all, they're just a sort of deflector to keep from overloading the axle seal with grease during a turn. I believe I've seen them, now that you mention it, but seldom.
Jerry
